1. Field of the Invention
The present invention relates generally to the field of scintillation camera and image display forming apparatus and more particularly to a method and apparatus for correcting for the non-uniformities of energy distribution of scintillation cameras resulting in shifts in the energy distributions as a function of camera image field position.
2. Description of the Prior Art
Distortion in the image from image data obtained from scintillation cameras and associated image display apparatus is primarily due to spatial distortion and variations in point source sensitivity (image event energy signal non-uniformities). In presently available scintillation cameras of the "Anger-type", variations in point source sensitivity are maintainable to within 1-2% by means of elaborate component specifications, tuning and adjustment. Further, spatial distortion correction methods are known and more accurate correction methods are becoming available to correct for image events not being recorded by the scintillation camera apparatus in their correct image event locations with respect to the overall image and their position of original occurrence.
For example, one spatial distortion correction method is disclosed in U.S. Pat. No. 3,745,345 which issued to G. Muehllehner on July 10, 1973. Another, more accurate spatial distortion correction method and apparatus is disclosed in copending U.S. application Ser. No. 051,176 filed by E. W. Stoub et al on June 22, 1979. This more accurate spatial distortion correction method provides accurate repositioning of each event and corrects spatial distortion to obtain a corrected image having density variations of as low as 1% in response to a uniform field flood source.
The arrival of more accurate spatial distortion correction in scintillation camera images results in an increased attention to scintillation camera problems and improvements in the area of non-uniformities caused by energy variations in the energy distribution of image event signals as a function of position on the camera face; also known as point source sensitivity variations and image event energy signal non-uniformities.
While it is possible to control the image event energy non-uniformities in energy distributions in scintillation cameras to within 1-2%, this control is not practical for efficient manufacture and the costs of manufacture are much increased due to elaborate component specification and tuning procedures that must be performed. Further, the requirements for scintillation camera components such as the scintillation crystal of the camera detector head are rather stringent and the result is an increased cost of the crystal and a larger than desirable reject rate of the crystals.
Thus, appropriate design, component specifications and tuning can reduce energy non-uniformities to 1-2% for either slowly varying non-uniformity characteristics across the face of the crystal or constantly varying characteristics across the crystal face. However, crystal non-uniformities that exhibit either abrupt changes within small areas of the crystal or discontinuities are not capable of being tuned out or compensated for during manufacture.
When, elaborate tuning is utilized as one element to obtain uniformity of the image event energy signals across the camera face, the camera is susceptible to detuning effects during the life of the camera such as to require frequent retuning during field life with resultant inconvenience and/or inaccurate image displays.
Another problem with the design, manufacture and tuning of the camera to achieve a low non-uniformity characteristic is the trade-off between camera characteristics such as point source non-uniformities, sensitivity, spatial distortion and resolution. The camera design characteristics that are required to achieve desirable uniformity in energy distribution across the image field of the camera result in the degradation of other important camera characteristics.
Scintillation cameras with corrected image density variations of 1% due to spatial distortion characteristics are only of value for diagnostic use when the non-uniformities due to energy variation are also accurately corrected since the variations in the image event energy signal if excessive cause an image event to be discarded if the image event energy signal does not fall within the energy window of the scintillation camera. The provision of a wider energy window of image event acceptance can result in the acceptance of image events that are caused by scattered radiation while still not achieving uniform acceptance of image events across the camera face.
Various methods to correct for the non-uniformities of scintillation cameras have been proposed. These methods are based on various attempts to vary the energy window against which each image event signal is compared to determine if the image event signal will be processed as a valid image event and displayed in accordance with the outputted X,Y image coordinates. These methods are sometimes referred to as "sliding energy window" techniques.
One method of energy window variation is described in a paper entitled "Removal Of Gamma Camera Non-Linearity And Non-Uniformities Through Real Time Signal Processing" by G. F. Knoll et al as presented at the Nuclear Medicine conference in Paris, France in July 1979.
The upper and lower extremes of a Z (energy) acceptance window are stored in a digital format at camera face array positions in a 64.times.64 format. Thus, when the X,Y position of an image event is produced by the camera, the corresponding upper and lower extremes of the acceptance window are read-out to a window comparison stage wherein the Z image event signal is compared to the programmed energy window.
Other studies discussing the use of non-uniformity corrections are "Sources of Gamma Camera Image Inequalities", Morrison et al, Journal of Nuclear Medicine 12: 785-791, 1971, and "A New Method Of Correcting For Detector Non-Uniformity In Gamma Cameras", Lapidus, Raytheon Medical Electronics, ST-3405, November, 1977.
The Lapidus article describes a method for modifying the Z (energy) signal of an image event by reading out a stored correction factor (from a 64.times.64 array) corresponding to the image event position. The correction factor varies the pulse width of the Z signal via a pulse width modulator to supply a variable pulse width Z signal. The display apparatus utilizes the variable pulse width Z signal to vary the intensity of the displayed image point on film. A flood mode is utilized to "learn" the distribution of non-uniformities. An array of values that represent the flood response of the camera detector at each of 4096 X-Y locations is then obtained and stored.
While the aformentioned prior art proposals and methods are generally suitable for their intended purpose and for studying non-uniformity response and effects, the previous systems, methods and proposals do not provide for modification of the energy of each image event signal to correct for non-uniform camera response and process the modified image event energy signals by a fixed energy window analyzer. Further, the prior art does not provide a system that is capable of automatically operating with different sources to correct for camera non-uniformities or with sources having multiple energy levels.